Telephoto lenses or lenses having relatively long focal lengths and relatively narrow fields of angular coverage are well known in the prior art. Some of the prior art telephoto lenses provide for internal focusing of a lens element or a group. In general, prior art telephoto lenses employing an internal focusing means have proved unsatisfactory in terms of weight and bulk of the lens. However, U.S. Pat. No. 4,359,272 discloses a compact, lighter internal focusing telephoto lens. The difficulty of obtaining the satisfactory compactness has been exacerbated by the extent of the movement of the internal focusing components which is required in order to obtain the desired focusing range within the overall optical design parameters.
Other deficiencies in prior art telephoto lenses having internal focusing components have been experienced with the necessity of providing internal focusing components which have a relatively large refractive power. The relatively large power is needed to minimize the total travel of these components. But is also presents problems with respect to aberrational corrections of the lens as a whole, both as to monochromatic and chromatic aberrations.
U.S. Pat. No. 4,068,929 discloses an internal focusing rear lens in which a positive component of a negative rear group is movable for focusing.
If the power of that focusing group is increased, the necessary correction of the various field dependant aberrations like distortion, astigmatism, etc., may become very difficult to achieve.
The present invention provides a new and improved internal focusing telephoto lens which is also adapted to have the focusing group driven by a motor in response to a signal received from a focus detector.